The superlative of the adjective is formed by adding -st, as is usually the  case in English too, and umlauting the preceding vowel where appropriate, e.g

billig  der/die/das billigste = > cheap  the cheapest
gesund  die/die/das gesündeste = > healthy  the healthiest
groß  der/die/das größte = > big  the biggest
klein  der/die/das kleinste = > small  the smallest

Adjectives ending in -d, -t or any s-sound (i.e. -s, -ss, -ß, -sch or -z) insert  an e before adding -st, e.g.

hübsch  Elke ist das hübscheste Mädchen in der Klasse.
pretty  Elke is the prettiest girl in the class

The same applies to the adjective neu (new), i.e. der neueste/am  neuesten. Adjectives of more than two syllables in English employ ‘most’ instead of  the ending ‘-st’, but this is not the case in German where -st can be added  to an adjective whatever the number of syllables it contains, e.g.

langweilig  der langweiligste Film
boring  the most boring Film

interessant  das interessanteste Buch
interesting  the most interesting book